Prefer RTF when we have both RICHTEXT and RTF.

Change-Id: Ib4133ae4fdecc32429d89b56b0c9466dd3451522
Reviewed-on: https://gerrit.libreoffice.org/47316Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>